This invention concerns a packaging foil for foods and other everyday goods, the foil being composed of two or more barrier layers as well as plastic coatings between the barrier layers and which form the outer surfaces of the foil; in addition, the invention provides for a method of manufacturing the foil.
Closed or lined containers made of foil and often provided in one way or another with barrier layers are used for preserving foods and other everyday goods. Tubes in which the foil itself also constitutes the bearing structure are a very common group within foil packaging. The job of the barrier layer, for example aluminium metal, is to serve as a protective layer through which environmental substances are not allowed to penetrate into the container, and vice versa.
Since most metals corrode, they should not be permitted to come into contact with the substance to be preserved but should be protected. Protection of the metal is generally provided by a plastic or protective lacquer film which is spread on, say, the aluminium foil when the foil is being manufactured. Owing to considerations of dyeing, weight and the like, some kind of protective coating is applied to both sides of the foil. Several manufacturing methods and substances are known in the production of foil, for example, Finnish patent applications Nos. 820247 and 821686 and U.S. Pat. No. 3,817,427.
All known methods of manufacturing foil are based on the idea of spreading an inexpensive and soft bulk polymer, most often polyethylene, in a sufficiently thick layer on the metal. A requirement of the protective layer is that it be thick, owing to sealing considerations and the like, since there must be a sufficient amount of sealing compound in the hot sealing operation. A reason for the fact that this drawback has not been corrected previously is the cheapness of bulk plastic. The view has been held that the seal of a foil casing requires a thick, readily melting plastic film in order to achieve adequate seal strength and sufficient stiffness of the foil.
In a number of packages such as tubes, the irreversibility of the forms of the compressed foil package is important. This can only be achieved with a foil provided with one metal barrier layer by making the barrier layer unduly thick so that its contours will not spring back due to the resilience of the plastic layers.
Generally known are foils based on the "sandwich" principle and incorporating several barrier or supporting structures. Such foils have been presented, for instance, in the German patent publications Nos. OS-DE 30 24 725 and OS-DE 31 13 428. These patents are nevertheless directed at specially stiff, self-supporting foils whose method of manufacture involves a laborious and multiphase coating layer by layer. They are not suited for making and mass producing the kind of packages in which the foil is folded and sealed in a manner such as the hot sealing process. No effort has been made to obtain savings of material in these solutions; rather, they seek to improve structural strength.
The foil edge remaining inside the container to be formed is also problematical in the known solutions because the edge of the barrier layer should not in most cases be allowed to enter into contact with the substance to be preserved. It has been attempted to solve this problem by means of an extra hot sealing treatment with an amount of fluid plastic as well as by "concealing" the edge under the fold. The first mentioned method is unsure and calls for the use of even thicker plastic films; the second mentioned method is serviceable but entails special measures prior to sealing.